The standard auto-focusing methods currently used in point-and-shoot digital cameras are collectively called “depth from focus.” In the depth-from-focus method, the whole range of focus positions is scanned (from infinity to the closest possible distance). At each focus position, an image is taken, and a metric quantifying the sharpness of the region in the image on which the camera is to be focused is calculated. The focus position having the highest sharpness metric is then used for acquiring the still image. Some kind of gradient operator is usually employed to define the sharpness metric.
The second class of auto-focus methods is collectively called “depth from defocus.” Unlike depth-from-focus, depth-from-defocus is not used in digital cameras, but is used in academic applications, such as optical measurement instruments or astrophysics. Depth from defocus is a method that estimates the depth map of a scene from a set of two or more images of the scene taken from the same point of view. The images are obtained by varying the camera parameters (typically the focus position, the zoom position, and/or the aperture size/iris). The information about the distance to the object is contained in the blur quantification of the defocused images.